Physical Science
Chapter 3: The Structure of Matter
Name:________________________
Chapter 3, Lesson 4: How Do Scientists Know What Atoms Are Like?
Section Goal: The student will describe what a model is and explain how scientists use it, explain how models
of the atom have changed and describe the electron cloud model.
Vocabulary:
1. Model2. Nucleus3. Proton4. Electron5. NeutronLesson 4: Notes

Since atoms are too small to be seen with the eyes alone, people have wondered for a long time what atoms look
like. Scientists have been studying atoms since the 1800’s. How do they know what an atom looks like if they
can’t see them?
Using Models
 Sometimes scientists can tell what things look like by studying how they act. Ex: Can you see the wind? We can
see the effects of the wind, like leaves blowing around, but we can’t actually see the wind. So, we use evidence.
 Scientists use this same evidence to study things they can’t see. Ex: By studying how atoms act, scientists decide
what an atom must look like.
 They use models (an idea, a picture or an object that is built to explain how something else looks or works) to
help people understand the way the objects acts.
Models of Atoms
 Scientists use models of atoms to show how atoms look and act without having to actually see them. Models have
been developed over 2000 years ago. As they gather new information about atoms, scientists change their
models.
 In the early 1900’s, scientists developed a model of an atom like this one. However, as scientists know more
about atoms, this kind of model is still useful for describing atoms.
Hydrogen

Helium
Find the center of each atom. This central part of an atom is called a nucleus. The nucleus of an atom contains
small particles called protons. Protons are labeled with the letter p. Another symbol for a proton is a plus (+)
sign. Electrons use the letter e or use the minus (-) sign. Electrons are particles in an atom that move around the
outside of the protons and are smaller than the protons. The protons and electrons of an atom stay together
because they attract each other.

In 1932, scientists had evidence that the nucleus of an atom had another kind of particle. This particle is call a
neutron. It is about the same size as a proton. Because of this new evidence, scientists changed the model of the
atom.

You can see that the electrons seem to be on a certain path around the nucleus of the atom. Scientists thought that
electrons moved in different layers around protons, sometimes jumping from one layer to another.
Today scientists use another model of atoms. It is called the electron cloud model. The dark center represents the
nucleus. However, you can’t see different layers of electrons like you saw in the first models.



This model was developed as a result of scientists gaining evidence that electrons behave in a more complicated
way than they previously thought. So, they are not sure how electrons move around the nucleus. As they learn
more, the model will most likely change.
You have looked at models showing the number of protons and electrons in the atoms of a few different elements.
On the following table, you will see some other elements listed with the numbers of protons and electrons in each.
What do you notice about the number of protons and the number of electrons in each of the elements?
Element
Hydrogen
Helium
Lithium
Beryllium
Boron
Carbon
Nitrogen
Oxygen
Fluorine
Neon

Number of protons
1
2
3
4
5
6
7
8
9
10
Number of electrons
1
2
3
4
5
6
7
8
9
10
The number of protons in an atom is equal to the number of electrons in the atom.
Lesson Reflection:
 Making Models of Atoms Investigations
Self-Check:
1. If scientists cannot see atoms, how do they know what they look like?
2. How do scientists use models?
Technology/Application/Connection to real-world: